Pending U.S. application Ser. No. 469,851, filed on May 14, 1974 discloses a time-temperature integrating indicator. The device disclosed in said application includes an elongated envelope of upper and lower walls of gas impermeable material which are heat sealed together in a continuous course extending about the periphery of each, with the walls further having a transverse cross-seal intermediate the ends of the envelope so as to define first and second compartments in the envelope. A gas generating material is disposed in the first compartment and wick means extends from the first compartment to the second compartment and intervenes the cross-seal whereby the wick means is the only gas communication between the first and second compartments. Further an indicator composition is deposited on the wick with the indicator composition producing a color change in the presence of gas generated by the gas generating material. The function of such device is to provide a temperature history, e.g., of a product associated with the device in visual display as a color front on the indicator wick with the distance of the front advancement being a function of the temperature time interval, the visual display serving to inform a user whether or not the product has been unduly subjected to adverse temperature conditions as might affect the ultimate product usage. The device is particularly suited for providing indication of the condition of foods, films, pharmaceuticals, biological preparations and the like to give indication of any decomposition, deterioration or change in the sterile character of such products.
While the device disclosed in said pending application functions well and accurately in many monitoring applications, there are certain utilizations wherein the device does not provide optimum results because the cross seal is not effective in all situations to prevent gas communication from the first compartment to the second compartment except through the wick means. As a practical consideration, it is not possible to effect a cross-seal of the envelope that does not leave small passages at either side of the wick. Thus gas transport from the first to the second compartment can occur without the gas contacting the wick in the region of the cross-seal. If there exists free passage space in the second compartment of any significant lateral expanse, as for example as depicted in FIG. 1 of said patent application, random gas molecules may transport through the second compartment to a location well beyond the cross-seal before first contacting the wick means. Also, the previous design is not effective in preventing gas escaping from the wick before reacting, reentering the wick at some location leading the indicator front, and reacting on the wick at this location. In either event, the indicator in the wick will change color at such location possibly giving an erroneous indication, since the indication front should advance progressively along the wick in a direction away from the cross-seal as a direct function of the time-temperature integral to which the device has been subjected. In other words, absorption of gas by the wick and reaction on it desirably should be confined to occurring progressively from the wick end nearest the gas source.